AUGUST 2011 Z H A N G E T A L . 2523 On the Characteristic Height Scales of the Hurricane Boundary Layer JUN A. ZHANG Rosenstiel School of Marine and Atmospheric Science, University of Miami, and NOAA/AOML/Hurricane Research Division, Miami, Florida ROBERT F. ROGERS NOAA/AOML/Hurricane Research Division, Miami, Florida DAVID S. NOLAN Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida FRANK D. MARKS JR. NOAA/AOML/Hurricane Research Division, Miami, Florida (Manuscript received 26 October 2010, in final form 9 March 2011) ABSTRACT In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations. 1. Introduction (PBL) parameterization schemes (e.g., Braun and Tao 2000; Nolan et al. 2009a,b; Smith and Thomsen 2010). The boundary layer is known to play an important role Understanding of the hurricane boundary layer structure in the energy transport processes of a hurricane, regulating has become increasingly important in the ongoing effort the radial and vertical distributions of momentum and toward developing high-resolution numerical models to enthalpy that are closely related to storm development improve hurricane intensity forecasts (e.g., Marks and Shay and intensification (e.g., Ooyama 1969; Emanuel 1986; 1998; Rogers et al. 2006; Chen et al. 2007; Davis et al. 2008). Wroe and Barnes 2003; Smith et al. 2008; Rotunno et al. In many PBL schemes used in full-physics numerical 2009; Smith and Montgomery 2010). Numerical studies models, one of the crucial elements is the determination of have shown that the simulated hurricane intensity is very the atmospheric boundary layer height (H), because it is sensitive to the selection of planetary boundary layer coupled with the maintenance of low-level clouds and energy transport from the surface layer to the boundary layer above (e.g., Troen and Mahrt 1986; Hong and Pan Corresponding author address: Dr. Jun Zhang, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 1996; Vogelezang and Holtslag 1996; Beljaars and Viterbo Rickenbacker Cswy., Miami, FL 33149. 1998; Noh et al. 2003). The boundary layer height is also E-mail: [email protected] a key variable that regulates the vertical distribution of DOI: 10.1175/MWR-D-10-05017.1 Ó 2011 American Meteorological Society Unauthenticated | Downloaded 09/30/21 09:25 PM UTC 2524 MONTHLY WEATHER REVIEW VOLUME 139 turbulent fluxes and helps determine where turbulent has been used in previous studies investigating the hurri- fluxes tend to become negligible (Stull 1988). cane inner-core structure (Jorgensen 1984; Frank 1977a,b, Despite the importance in defining the boundary layer 1984), boundary layer structure (Franklin et al. 2003; top in hurricane models, there has been no consensus on Powell et al. 2003), and surface layer air–sea thermal what should define this top in the hurricane research com- structure (Cione et al. 2000). The advantage of the com- munity. In the slab model used in the seminal theoretical posite analysis is that it provides a general picture and hurricane model of Emanuel (1986), a constant boundary characterization of the fields that are investigated. In this layer height is used. Early boundary layer studies (e.g., case, we intend to improve our understanding of the Powell 1990; Anthes and Chang 1978) adopted a thermo- mean boundary layer structure in hurricanes in terms of dynamic definition of the boundary layer, characterized by the boundary layer height. The most important drawback the layer in which the potential temperature or virtual to compositing is that it tends to smooth the data from potential temperature is appreciably well mixed. The ther- a large number of storms that may not be similar (Frank modynamic definition is mainly based on one observa- 1977a). The success of a compositing analysis depends on tional study of the boundary layer of Tropical Storm Eloise the similarity of the events studied. For this purpose, only (1975) by Moss and Merceret (1976), who found that mo- sondes in storms of at least hurricane intensity (.64 kt; mentum fluxes tend to become near zero near the top 1 kt = 0.5144 m s21) are used in the analysis. of the mixed layer defined using the potential tempera- The data are grouped as a function of the radius to ture profile, similar to the vertical flux profile in a typi- the storm center (r) that is normalized by the radius of cal tropical boundary layer over the ocean. Bryan and the maximum wind speed (RMW; i.e., r* 5 r/RMW). The Rotunno (2009) define the top of the boundary layer to be center positions have been determined using the flight the height of the maximum wind usually around 1 km. level to fix the storm center using the algorithm de- Kepert and Wang (2001, see their Fig. 2) show that the veloped by Willoughby and Chelmow (1982). Values of stress divergence becomes small near the height of the RMW are mainly determined using the Doppler radar maximum wind or azimuthal jet, similar to Bryan and data from the tangential winds at 2 km. When there are Rotunno’s result. Smith et al. (2009) adopt another dy- no radar data available, the RMW is determined from the namical definition, considering the strong inflow layer as flight level data. When compositing the data, the radial the boundary layer because of the frictional disruption of bin width is 0.2r* for the inner core (r* , 2), and it is 0.4r* the gradient wind balance near the surface (see their Fig. 6). for the outer part. The data are also bin averaged verti- The purpose of this paper is to use observational data cally at 10-m resolution. The final averaged data are also from multiple hurricanes to examine the structure of the smoothed using a simple 1–2–1 filter both vertically and hurricane boundary layer. We focus on investigating the horizontally, repeated 5 times. characteristic height scales of the hurricane boundary b. Data coverage and quality control layer through analyses of 794 global positioning system (GPS) dropsonde data collected by National Oceanic The dropsonde data used in this study were collected and Atmospheric Administration (NOAA) research by a total of 106 NOAA research flights in 13 hurricanes aircraft in 13 hurricanes. As part of NOAA’s Hurricane (Table 1). A detailed description of the instrumentation Forecast Improvement Project (HFIP), this work also related to the dropsonde can be found in Hock and builds a dataset that can be used to evaluate the repre- Franklin (1999). The fall speed of a sonde is 12–14 m s21, sentation of boundary layer structure in model simula- while the typical sampling rate is 2 Hz, providing mea- tions. Section 2 describes the data sources and analysis surements with 6–7 m of separation in the vertical on method, and includes a detailed description of how dif- average. The dropsonde gives measurements of air tem- ferent hurricane boundary layer heights are defined. In perature, relative humidity, pressure, and horizontal section 3, we present the results by comparing different and vertical wind speed. Typical measurement errors boundary layer height scales determined using our data for pressure, temperature, and relative humidity are to those from previous studies. Section 4 summarizes the 1.0 hPa, 0.28C, and 5%, respectively (Hock and Franklin results and discusses future work. 1999). The accuracy of the horizontal wind speed mea- surements is 2.0 m s21 and ,0.5 m s21 for the verti- cal winds with approximately 0.2 m s21 precision. The 2. Data and methodology dropsonde data have been processed and quality con- trolled using the EDITSONDE software developed by a. Analysis method the Hurricane Research Division (Franklin et al. 2003). The dropsonde data are analyzed and grouped within Data from 2231 GPS dropsondes have been processed a composite framework. The composite analysis technique and analyzed. However, only 794 of these that have Unauthenticated | Downloaded 09/30/21 09:25 PM UTC AUGUST 2011 Z H A N G E T A L . 2525 TABLE 1. Storm information and number of sondes. Storm intensity Storm Year range (kt) No. of sondes Erika 1997 83–110 40 Bonnie 1998 68–93 76 Georges 1998 66–78 39 Mitch 1998 145–155 28 Bret 1999 75–90 33 Dennis 1999 65–72 7 Floyd 1999 80–110 40 Fabian 2003 68–120 131 Isabel 2003 85–140 162 Frances 2004 68–83 62 Ivan 2004 65–135 123 Dennis 2005 65–70 7 Katrina 2005 68–100 46 continuous measurements of wind speed, temperature, and humidity from the flight level to the surface (10 m) FIG.